The detection of acoustic waves using optical detection is becoming important because of the considerable improvement in sensitivity and absence of direct electrical contact. The typical examples are Fabry-Perot (FP) resonator and ring resonator.
The FP resonator provides an effective way for optical detection of acoustic waves. The FP resonator involves a layer of polymer sandwiched between one highly reflective metal layer and one partially reflective metal layer. The incident beams of light propagate into the FP resonator and the reflected beams of light are emitted from the FP resonator. As the acoustic waves hit the resonator, the polymer layer deforms under the influence of the waves, thus perturbing the resonant response of the structure. The polymer layer is commonly used because of its elasticity, so that it responds with a large dimensional change for a given imposed pressure. Pressure sensitivities around 200 Pa in 20 MHz bandwidth are reported for FP resonators. Most of the FP resonators use metal reflectors, although there have been other attempts to increase their quality factor by using lossless photonic crystal reflectors, these multilayer structures are still relatively complex.
High sensitivity for acoustic detection has been achieved by the polymer ring resonator, where the sound beam distorts the resonator so that the effective index of the guided mode is perturbed resulting in a change of the resonant wavelength. An impressive sensitivity of noise equivalent pressure of 6.8 Pa in a 140 MHz bandwidth can be provided. Nevertheless, the disadvantages of the ring resonator are that it is hard to provide large scale parallel operation, and its operating dynamic range is limited.
A need therefore exists for a novel resonator with the improved performance to eliminate or diminish the disadvantages and problems described above.